This invention relates to a machining method for reducing an amount of projection of one of two protuberances (the expression xe2x80x9creducing an amount of projection of a protuberance(s)xe2x80x9d used herein includes not only a concept of diminishing an amount of projection but also a concept of eliminating the protuberance(s)) by spinning, which protuberances are formed on intersection parts, in a short axis, between a flattened tube stock part of a sectional configuration having a short axis and a long axis and a reduced-diameter part of a circular sectional configuration which is formed at one end portion of the tube stock part by spinning.
Heretofore, an elliptic tube (special-shaped tube) having an elliptic configuration in section has been widely used for a catalyst converter and a premuffler of automotive vehicles. The reasons are as follows. Comparing the elliptic tube with the circular tube having a circular configuration in section, if the sectional areas of them are same, the length of the elliptic tube in short axis direction is shorter than the diameter of the circular tube. Therefore, in the case where the catalyst converter, etc. are loaded under the floor (i.e., chassis) of a vehicle, the elliptic tube has such an advantage that by loading the catalyst converter, etc. such that the short axis direction of the elliptic tube is oriented to an up and down direction, the distance from a ground surface to the elliptic tube can be increased.
Incidentally, an exhaust tube is connected to opposite end portions of the catalyst converter and the premuffler. The exhaust tube normally has a circular configuration in section. So, as shown in FIG. 8, reduced diameter portions 2 each comprising a tapered section 2a and a straight tube section 2b are formed on opposite end portions of the elliptic tube 1 which constitutes the catalyst converter and the premuffler, and the exhaust tube is connected to the straight tube section 2b. 
In general, the reduced diameter portion 2 is, as shown in FIG. 9, formed by spinning. Spinning against the reduced diameter portion 2 is executed by press contacting a roller (forming tool) R with the end portion of the elliptic tube 1 while revolving the roller R about an axis L of the elliptic tube 1. While revolving the roller R, the roller R is moved from a center side of the elliptic tube 1 to the end portion side so that the revolution radius of the roller R is reduced. By this, the tapered section 2a is formed. Thereafter, while keeping the revolution radius of the roller R fixed, the roller R is moved to an end face of a tube stock part 3, thereby forming the straight tube section 2b (see Japanese Patent Application Laid-Open No. H03-226327).
In the elliptic tube 1, the outer periphery of the tube stock part 3 which is not yet subjected to spinning is, as shown in FIG. 10, preferably smoothly continuous with the outer periphery of the tapered section 2a of the reduced diameter portion 2 which is already subjected to spinning. However, if the reduced diameter portion 2 is subjected to spinning, protuberances 4 are, as shown in FIGS. 8(A) to 8(C), formed on two intersection parts in the short axis direction of all the intersection portions between the outer peripheral portion of the tube stock part 3 and the outer peripheral portion of the diameter reduced portion 2. The reason is that the spinning executed by the roller R starts from the outer peripheral portion of the tube stock part 3 in the long axis direction and therefore, as indicated by arrows of FIG. 9, the metal composition of a portion forming the long axis side of the tube stock part 3 is brought closer towards the short axis side as the roller R revolves. As a result, the protuberance 4 is formed. The formation of the protuberance 4 brings about such a problem that the protuberance 4 interferes with other members at the time of mounting the catalyst or premuffler in which the elliptic tube 1 is used on a vehicle. In case of the converter, for example, the protuberance 4 interferes with the heat insulation cover. Therefore, it is demanded to develop a machining method for either eliminating the protuberance 4 or diminishing the amount of projection thereof.
The present invention has been accomplished in order to meet with the above-mentioned demand. According to the present invention, there is provided a machining method for reducing, by spinning, an amount of projection of one of two protuberances of a special shaped tube including a flattened tube stock part of a sectional configuration having a short axis and a long axis and a reduced-diameter part of a circular sectional configuration which is formed at one end portion of the tube stock part by spinning which is executed by revolving a forming tool with respect to the tube stock part, the two protuberances being formed on two intersection parts between an outer peripheral portion in the short axis direction of the tube stock part and an outer peripheral portion of the reduced diameter part and projecting outward in the short axis direction, a revolution center line of the forming tool for the spinning with respect to the special shaped tube being made eccentric towards the other of the two protuberances from a center between the two protuberances, and the forming tool being press contacted with only the one of the two protuberances located on the opposite side to the eccentric direction so as to spin only the one of the two protuberances.
It is preferred that an eccentric amount from the center between the two protuberances to the revolution center line of the forming tool is set such that when a revolution radius of the forming tool is gradually reduced from a state in which the forming tool is not in contact with any part of the elliptic tube, the forming tool contacts said one protuberance generally first.
The forming tool is preferably moved in a direction of the revolution center line and particularly preferably reciprocally moved in a direction of the revolution center line plural times. In the case where the forming tool is reciprocally moved plural times, it is preferred that the forming tool is moved by only a predetermined distance towards the revolution center line side so that the revolution radius is reduced at least at one end portion in a direction of the reciprocal movements.